In-line/on demand reaction delivery system and method

ABSTRACT

An on-line method for the continuous generation of alkali metal salts includes continuously reacting a source of acidity, a liquid having a low pH value, with a source of alkalinity, a liquid having a high pH value, while continuously monitoring an electrical parameter of the resultant in salt which is indicative of its pH value. The monitored value is used to adjust the relative flow rates of the input liquids so as to dynamically maintain the pH value of the resultant salt within a predetermined range. In an apparatus, the input liquids can be continuously supplied to a reacting element and the conductance of the resultant salt monitored for purposes of providing feedback control signals to adjust one or more flow rates.

FIELD OF THE INVENTION

The invention pertains to processes and methods for producing alkalimetal salts. More particularly, the invention pertains to apparatus andmethods for in-line high volume and continuous production of saltsusable as emulsifying agents in food products.

BACKGROUND OF THE INVENTION

The use of alkali metal salts as emulsifying agents in connection withthe processing of food products is known. Representative emulsifyingagents include sodium phosphates, polyphosphates and citrates which areoften used in connection with the manufacture of process cheese.

It is known to use monosodium phosphate (NaH₂ PO₄) (MSP), disodiumphosphate (Na₂ HPO₄) (DSP) as well as trisodium phosphate (Na₃ PO₄)(TSP) in connection with process cheese manufacturing. Similarly, it isknown to use sodium and potassium citrates as emulsifying agents inconnection with the manufacture of dairy products and process cheese.

The publication Process Cheese which bears a copyright date of 1992,published by D. Cooley & Co., Inc. describes, Page 47 and 48, theprocess of titrating phosphoric acid with a base so as to producevarious salts. More particularly, MSP, DSP or TSP or blends thereof canbe manufactured in accordance with the teachings of the above-notedpublication by reacting phosphoric acid as (H₃ PO₄) well as MSP, DSP,TSP or blends thereof with a source of alkalinity such as sodiumhydroxide, sodium carbonate or mixtures thereof.

As is known, the process involves a sequential substitution of ahydrogen atom with a sodium atom to produce respectively MSP, DSP andTSP. It is also known to carry out the reaction in distinct batch steps.

Phosphoric acid can be reacted with sufficient sodium hydroxide toproduce MSP. The MSP can be stored in liquid form at the premises of thesalt manufacturer or at the premises of the cheese processor.

The liquid MSP can be withdrawn from a storage tank and reacted withadditional sodium hydroxide on the batch basis to produce DSP or TSP ora blend thereof. The batch produced salt, at an appropriate pH value canthen be introduced into the cheese processing as is known to those ofskill in the art.

Batch processing of DSP and TSP, or blends thereof, while useful andeffective in cheese processing, inherently has limitations with respectto smoothness of texture, flavor, metalability, softness, sliceseparation and the like in the final cheese product. It would bedesirable to overcome and improve upon the known process.

Thus, there continues to be a need for improved devices and methods ofcreating emulsifying agents useable as food additives so as to improvethe uniformity and consistency of the final processed food product.Preferably, such devices and methods would be incorporatable intoexisting manufacturing systems, used for example, to manufacture processcheese, without having to provide extensive and expensive additionalprocessing equipment. Preferably, such improved devices and methodswould also contribute to a reduction in the manufacture of excessiveamounts of emulsifying agents which in turn will help reduce the cost ofthe final process cheese product to the ultimate consumer.

SUMMARY OF THE INVENTION

In accordance with the invention, a system and a method are providedwhich incorporate in-line, continuous, real-time reacting of a selectedacid or liquids having low pH values with a source of alkalinity incombination with substantially continuous feedback and control of thecharacteristics of the resultant salts. These salts can in turn beprovided, on a continuous basis, to a food process. In accordance withthe present invention, the pH value of the resultant salt can becontrolled using a feedback loop by continuous, real-time monitoring ofan conductance parameter, indicative of pH of the resultant salt whichis simultaneously being provided to the food process. Temperature canalso be continuously monitored in real-time.

In one aspect of the invention, the system can include first and secondsources, or storage tanks, of an acid or a liquid having a selected,low, pH value and a source of alkalinity respectively. In accordancewith the present invention, both the initial source of acidity and thesource of alkalinity are in liquid form.

The present system provides for continuous, real-time reacting of thetwo starting liquids so as to carry out, for example, a selectedsequential hydrogen substitution with sodium (or perhaps potassium)ions. The reaction takes place in real-time on a continuous basis and inan in-line reacting element.

A selected electrical parameter indicative of pH, conductance, of theliquid outflow of the mixing element, is monitored in real time and usedto dynamically vary the volumes and/or rates of the input liquids so asto maintain continuously and in real-time, a desired pH value in theresultant output salt. In another aspect of the invention, the pH valuecan be maintained in a range of 4.1 to 12.2.

The output salt can then be immediately delivered, in real-time, to arespective food processing vessel. Alternately, the salt can bedelivered to a tank or the like for storage and subsequent use.

In accordance with the present system and method, acids such asphosphoric acid or citric acid or pre-processed, low pH liquids such asMSP, can be used as the input source of acidity to be reacted with arespective source of alkalinity in the on-line mixing element.

In yet another aspect of the invention, a programmable control unit,which might incorporate a microcomputer or the like, can be incorporatedinto the system for the purpose of carrying out a substantiallycontinuous sampling of the parameter value of the output salt. Outputsfrom the control unit can in turn be used on a real-time basis to adjustrates and flows of one or both of the input liquids.

As an alternate to programmable control units, hard-wired digital oranalog control systems could be used to implement the control process ona continuous or a sampling basis.

In yet another aspect of the invention, a plurality of controlparameters can be stored in a single control unit, such as aprogrammable processor so as to be able to provide, in response tooperator input, a plurality of different output salts. In addition todynamically controlling the characteristics and quantity of the outputsalts, in yet another aspect of the invention, real-time feedback can beprovided to an operator as to the status and condition of the system.

Numerous other advantages and features of the present invention willbecome readily apparent from the following detailed description of theinvention and the embodiments thereof, from the claims and from theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall block diagram of a system in accordance with thepresent invention;

FIG. 2 is a flow diagram of a method in accordance with the presentinvention;

FIG. 3 is a graph of pH vs. concentration for MSP, DSP, TSP;

FIG. 4 is a graph of conductivity vs. concentration for MSP, DSP, TSP,with curves drawn through measured data points;

FIG. 5 is a temperature compensation graph for compensation factors;

FIG. 6 is a temperature compensation graph for conductivity; and

FIG. 7 is a temperature compensation graph for compensation factor withlinear interpolation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

While this invention is susceptible of embodiment in many differentforms, there are shown in the drawing and will be described herein indetail specific embodiments thereof with the understanding that thepresent disclosure is to be considered as an exemplification of theprinciples of the invention and is not intended to limit the inventionto the specific embodiments illustrated.

FIG. 1 illustrates a system 10 in accordance with the present invention.The system 10 incorporates first and second storage units or tanks 12,14.

The tank 12 is, for example, used to store a source of liquid acidity"A" such as a citric acid, or a phosphoric acid. It can also be used tostore a partially processed starting liquid such as MSP.

The tank 14 is used for purposes of storing a source of alkalinity "B"such as sodium hydroxide or another suitable base.

Each of the tanks 12, 14 can be refilled as needed from a respectivetank truck or rail car T1, T2. Each of the tanks 12, 14 has associatedtherewith a respective constant pressure header loop 12a, 14a.

Each of the header loops, 12a, 14a includes an associated pump 12b, 14band a back pressure regulating valve 12c, 14c. The valves 12c, 14c canbe for example Model Number 6871/K21.

Other valves or control elements can be provided in connection with theheader loops 12a, 14a as would be known to those of skill in the art. Itwill be understood that the exact details of those header loops are notlimitations of the present invention.

The outputs from each of the respective header loops, via conduits 20aand 22a are streams of the two liquids A, B to be reacted together. Eachof the liquids is under a constant pressure determined by the respectiveheader loops 12a, 14a. The liquid streams are reacted under the controlof a closed loop feedback system 10'.

Each of the conduits is coupled to a respective flow control element20b, 22b in the system 10'. For purposes of describing the best mode andnot by way of limitation, flow control element 20b is a fixed flowcontrol element which provides delivery of a preset volume at a presetrate to a "T" 26.

An output port of the "T" 26 is coupled to a flow meter 30. Also coupledas an input to the "T" 26 is a variable flow control element 22b whichis in turn coupled to the conduit 22a.

Liquids A, B are combined in "T" 26, flow through the flow meter 30 andinto reacting element 32. Element 32 is a 12 inch or larger in-linemixer which provides, in effect, a dwell time to permit the desiredreaction to run to completion. In element 32, the liquids A and B reactso as to produce an output salt which is intended to have apredetermined pH value. Dynamic, real-time sampled or continuous controlover the output liquid from the mixing element 32 is achieved bymonitoring the conductance thereof in a conductivity measuring element36. One usable conductance meter is model 672E sold by Great LakeInstruments.

Surprisingly, the conductance parameter provides, as discussedsubsequently, a reliable, dynamic real-time indicator of the pH value ofthe output liquid. Other electrical parameters related to the pH valueof the output salt could be monitored without departing from the spiritand scope of the present invention.

An output signal from the measuring element 36 is coupled to a controlunit 40 which in turn provides input control signals to the variableflow control element 22b. The control unit 40 could include aprogrammable processor or hard wired digital or analog circuitry.

As the salt produced from the reaction of liquids A, B flows out of themixing element 32, the conductance thereof, whether sampled or measuredcontinuously and in real- time, provides appropriate feedback signals,which in turn produce control signals to the flow control element 22b tocontinuously produce a salt having the desired pH value in a selectedrange such as 4.1 to 12.2.

The output salt is coupled via a conduit 40 and appropriate valuing 42,as would be known to one of skill in the art, to one or more foodprocessing units 44a, b. These units could be cheese processors orcookers wherein the respective salt is combined with other foodprecursor ingredients to produce a desired food product. One possiblefood product producible with a system such as a system 10, is processcheese. Alternately, other ingredients can be combined with the saltseliminating from the reacting element 32 to produce different foodproducts without limitation. Further, the system 10 could be used toproduce non-food alkali metal salts for use in industrial processes.

As will be understood by those of skill in the art, multiple foodprocessing vessels, 46a . . . n can be operated in parallel usingmultiple closed loop control systems, comparable to the control system10' previously discussed. Where control unit 40 is programmable,multiple sets of parameters can be pre-stored to produce multipledifferent output salts in accordance with a selection by an operator.

FIG. 2 is a flow diagram illustrating the steps of a method inaccordance with the present invention. In an initial step 100 thedesired concentration of output additive or salt as well as desiredvolumes are entered perhaps via keyboard and display unit 40a. In a step102, a prestored table T1 is checked to establish whether or not thedesired concentration of the output salt is present in the table. Ifnot, the method returns to step 100 wherein a message could be providedvia the input output device 40a.

If the desired concentration is located in the Table T1, the table isaccessed and the corresponding conductivity set point is obtained in astep 106. In a step 108, temperature compensation information is thenobtained from a prestored Table T2.

Once the compensated set point value is obtained in the step 108, thecontrol unit 40 can then initiate the process by sending control signalson lines 40b and 40c respectively to the fixed flow control element 20band the variable flow control element 22b. At the same time the pumps12b and 14b for the respective constant pressure control loops 12a and14a are energized thereby producing the above described streams offluids A and B which come together at the "T" 26 and are in turn reactedin the reaction element 32 to produce the flow of output salts in theconduit 40. At the same time, in a step 114 the conductivity of theoutput salt is measured and fed back to the control unit 40 via a line40d. In a step 116, the conductivity value fed back on the line 40d iscompared to the previously established set point obtained from the step108. If the detected conductivity corresponds to the pre-established setpoint conductivity, no further adjustments are made to the variable flowdisplacement element 22b.

In the event that the detected conductivity does not correspond to theset point, in a step 118 a determination is made as to whether theconductivity is above or below the pre-established set point. If above,in a step 120, the variable flow control element 22b is adjusted apredetermined amount so as to reduce the flow of liquid B. Conversely,if the feedback conductivity value is below the pre-established setpoint, in a step 122 the variable flow control element control 22b iscaused to open a predetermined amount so as to increase the flow ofliquid B thereby raising the pH value of the output salt.

In step 126 a reading is taken of delivered output volume. The outputvolume of the produced salt could be delivered into one or more of thecookers, such as the cookers 44a, 44b or could be delivered to a storagetank for temporary storage before either further processing or use.

In a step 128, a determination is made based on the previously enteredvolume information as to whether the desired volume has been delivered.If so, the process can be terminated in an orderly fashion in a step130. If not, further readings of the conductivity of the output salt aretaken again in step 114 and the control process continues until thepre-established volume of output salt has been delivered.

By way of illustrating how the information for Table T1 is determined,reference is made to the graph of FIG. 3. In FIG. 3, pH values areplotted against percent solids for MSP, DSP and TSP. As illustrated inFIG. 3, there is a significant differential in pH values for the MSP andTSP. However, it has been determined that given the essentially zeroslope associated with the curve for DSP over the range of interest, inexcess of 30% solids, along with the difficulty of directly measuringsmall pH values in a range of 12 due to hydrolyzing of water by the TSP,along with a drift in performance of instruments for measuring small pHvalues, that direct measurement of pH values in the present circumstanceis an approach which is not preferred.

A preferred alternate, using conductivity measurements is illustrated byTables 1A, 1B and the graph of FIG. 4 (Conductivity vs. Percent Solids).

                  TABLE 1A                                                        ______________________________________                                        DISODIUM PHOSPHATE 41% (40.955%)                                              CHEMICAL FORMULA: Na.sub.2 HPO.sub.4                                          RAW MATERIAL   M.W.     ADJ. WT. % (WT./WT.)                                  ______________________________________                                        Monosodium Phosphate 45%                                                                     119.98   266.622  76.92                                        NaOH 50%       40.00    80.00    23.08                                        ______________________________________                                    

                  TABLE 1B                                                        ______________________________________                                        TRISODIUM PHOSPHATE 38.5% (38.4275%)                                          CHEMICAL FORMULA: Na.sub.3 PO.sub.4                                           RAW MATERIAL   M.W.     ADJ. WT. % (WT./WT.)                                  ______________________________________                                        Monosodium Phosphate 45%                                                                     119.98   266.622  62.496                                       NaOH 50%       40.00    160.00   37.504                                       ______________________________________                                    

As illustrated in Table 1A, a pre-determined amount of 45% MSP, at aknown conductivity, is reacted with a pre-determined amount of 50% NaOHat a ratio of MSP:NaOH of 3.333:1 (WT/WT). This in turn produces 41% DSPat a known conductivity.

As illustrated in Table 1B, a pre-determined amount of 45% MSP, againhaving a known conductivity is reacted with a pre-determined amount of50% NaOH. In this case, the ratio of MSP to NaOH is 1.666:1 (WT/WT).This results in a 38.5% TSP at a known conductivity.

As a result of the above-described steps, one obtains 45% MSP with aknown conductivity, 41% DSP with a known conductivity and 38.5% TSP witha known conductivity. These three phosphates are represented on thegraph of FIG. 4 as data points 1, 2, and 3. Given the above-notedratios, no other products will be present other than the aforementioned.

The three data points, as illustrated in FIG. 4, all lie substantiallyalong a straight line. Hence, adding 50% NaOH to 45% MSP results in aprocess of moving from data point 1, to data point 2, to data point 3.There is thus, a single conductivity reading for any phosphate ratiothat lies along the noted sliding conductivity line of FIG. 4. The datapoints noted in FIG. 4 are all located on regions of substantial slopefor each of the phosphates, thereby providing substantial and detectablevariations in conductivity for control purposes. This also makes itpossible to take into account variances in raw material concentrations.

Using the above-described process, set point information for Table T1can be established. Instead of prestoring Table T1, the equation off ofFIG. 4 could be executed each time a pH value is set, to determine therespective conductivity.

Temperature compensation values for Table T2 for a specific conductivitycan be established using predetermined temperature coefficients. Suchtemperature coefficients represent a percent change of measuredconductivity per degree Centigrade (°C.). In this regard, the measuredconductivity for a given electrolytic fluid will vary with thetemperature thereof. Such temperature coefficients can be linear ornon-linear functions.

In one aspect, for example, at 25° C. a pre-determined concentration ofDSP would have a measurable conductance characteristic. Increasing thetemperature by 10° C. will result in a different conductance parameter.

The difference between the two measured conductance parameters can beobtained. A first order temperature coefficient can be established bydividing the difference by ten (10) for that particular range of thatfluid. Further investigation would establish whether it was desirable touse a non-linear rather than a linear coefficient over the range.

Sample rates for the conductance measurement can, for example, be set tohave a period of 0.1 seconds and provide an acceptable quality of outputsalt.

FIG. 5 illustrates a substantially linear relationship betweencompensation factor and temperature for example, for three differentconcentrations of materials such as monosodium phosphate. Hence, a givenconductivity reading can be normalized, and the temperature effectsremoved, by multiplying the raw conductivity readings from the meter 36by a compensation factor associated with the temperature present in theoutflow from the mixing element 32.

FIG. 6 illustrates conductivity values as a function of temperature forthree different concentrations of MSP. As is illustrated in FIG. 6, asubstantially linear relationship exits between conductivity andtemperature. Tables 1-A, 1-B and 1-C illustrate the relationship betweenmeasured temperature, in the vicinity of the outflow port of mixingchamber 32 as well as the conductance meter 36 and the direct outputfrom the conductance meter 36 (labeled "raw") as well as an associatedtemperature compensation value for each of 10 temperatures in connectionwith each of the three different concentrations of MSP.

The information of Tables 1-A, 1-B, 1-C representing compensation valuevs. temperature can be stored in an appropriate table or tables forlook-up and usage whenever a conductance value is read off of the meter36. The meter 36 can, via the lines 40d read back either an adjustedconductance value or the raw conductance value as well as thetemperature in which case the control unit 40 can extract thetemperature coefficient associated with the sensed temperature, (from apre-stored table) and then multiply the raw conductance value by thetemperature compensation factor to obtain a normalized value independentof temperature.

FIG. 7 illustrates compensation factor vs. temperature where actualmeasure data points have been linerally related for interpolationpurposes.

Tables 2-A, 2-B and 2-C illustrate temperature readings vs. rawconductance readings wherein compensation values associated withtemperatures indicated with an asterisk have produced by interpolationas in FIG. 7. Thus, the linear relationship of compensation factor vs.temperature makes it possible to establish compensation values fortemperatures beyond those where measurements were actually made. Thus,using the above information, a table of temperature vs. compensationco-efficients can be stored in control unit 40 for purposes of adjustingthe raw conductance values received from conductance meter 36.

From the foregoing, it will be observed that numerous variations andmodifications may be effected without departing from the spirit andscope of the invention. It is to be understood that no limitation withrespect to the specific apparatus illustrated herein is intended orshould be inferred. It is, of course, intended to cover by the appendedclaims all such modifications as fall within the scope of the claims.

The above described system and method can maintain the pH value of theoutput salt in a range of ±1% relative to the set-point value.

                  TABLE 1-A                                                       ______________________________________                                        Batch #1                                                                      TEMP.              RAW    COMP.                                               ______________________________________                                        Pt. 1   82.5           200    1                                               Pt. 2   83.7           205.1  0.9751                                          Pt. 3   84.9           210.6  0.9497                                          Pt. 4   86.1           215.9  0.9263                                          Pt. 5   87.3           220.5  0.9070                                          Pt. 6   88.5           226.3  0.8838                                          Pt. 7   89.7           231.7  0.8632                                          Pt. 8   90.9           237.2  0.8417                                          Pt. 9   92.0           242    0.8264                                          Pt. 10  93.1           246.6  0.8110                                          ______________________________________                                    

                  TABLE 1-B                                                       ______________________________________                                        Batch #2                                                                      TEMP.              RAW     COMP.                                              ______________________________________                                        Pt. 1   82.4           202.80  1                                              Pt. 2   83.6           207.90  0.9755                                         Pt. 3   84.8           213.00  0.9521                                         Pt. 4   86.0           218.60  0.9277                                         Pt. 5   87.2           223.00  0.9094                                         Pt. 6   88.4           228.40  0.8879                                         Pt. 7   89.6           233.70  0.8678                                         Pt. 8   90.8           238.90  0.8489                                         Pt. 9   92.0           244.10  0.8308                                         Pt. 10  93.2           249.20  0.8138                                         ______________________________________                                    

                  TABLE 1-C                                                       ______________________________________                                        Batch #3                                                                      TEMP.              RAW     COMP.                                              ______________________________________                                        Pt. 1   82.5           202.7   1                                              Pt. 2   83.7           207.6   0.9764                                         Pt. 3   84.9           212.9   0.9521                                         Pt. 4   86.1           218.2   0.9290                                         Pt. 5   87.3           222.8   0.9098                                         Pt. 6   88.5           228.2   0.8883                                         Pt. 7   89.7           233.6   0.8677                                         Pt. 8   90.9           238.9   0.8485                                         Pt. 9   92.1           244.10  0.8304                                         Pt. 10  93.0           248.0   0.8173                                         ______________________________________                                    

                  TABLE 2-A                                                       ______________________________________                                        Batch #1                                                                      TEMP.              RAW     COMP.                                              ______________________________________                                        Pt. 1   82.5           200     1                                              Pt. 2   83.7           205.1   0.9751                                         Pt. 3   84.9           210.6   0.9497                                         Pt. 4   86.1           215.9   0.9263                                         Pt. 5   87.3           220.5   0.9070                                         Pt. 6   88.5           226.3   0.8838                                         Pt. 7   89.7           231.7   0.8632                                         Pt. 8   90.9           237.2   0.8417                                         Pt. 9   92.1*          242.4   0.8251                                         Pt. 10  93.3*          247.0   0.8097                                         ______________________________________                                         *Adjusted linear interpolation                                           

                  TABLE 2-B                                                       ______________________________________                                        Batch #2                                                                      TEMP.              RAW     COMP.                                              ______________________________________                                        Pt. 1   82.5*          203.2   1                                              Pt. 2   83.7*          208.3   0.9755                                         Pt. 3   84.9*          213.5   0.9518                                         Pt. 4   86.1*          219.0   0.9278                                         Pt. 5   87.3*          223.6   0.9088                                         Pt. 6   88.5*          228.8   0.8881                                         Pt. 7   89.7*          234.1   0.8680                                         Pt. 8   90.9*          239.3   0.8491                                         Pt. 9   92.1*          244.6   0.8307                                         Pt. 10  93.3*          249.6   0.8141                                         ______________________________________                                         *Adjusted linear interpolation                                           

                  TABLE 2-C                                                       ______________________________________                                        Batch #3                                                                      TEMP.              RAW*    COMP.                                              ______________________________________                                        Pt. 1   82.5           202.7   1                                              Pt. 2   83.7           207.6   0.9764                                         Pt. 3   84.9           212.9   0.9521                                         Pt. 4   86.1           218.2   0.9290                                         Pt. 5   87.3           222.8   0.9098                                         Pt. 6   88.5           228.2   0.8883                                         Pt. 7   89.7           233.6   0.8677                                         Pt. 8   90.9           238.9   0.8485                                         Pt. 9   92.1           244.10  0.8304                                         Pt. 10  93.3*          249.3   0.8131                                         ______________________________________                                         *Adjusted linear interpolation                                           

The invention claimed is:
 1. An apparatus for production of a selectedsalt comprising:a source of a first liquid having a relatively low pHvalue; a source of a second liquid having a relatively high pH value; areaction chamber with an output port; flow conduits for coupling the twosources to the chamber; at least one flow control mechanism forcontrolling a flow from one of the sources into the chamber; a controlunit coupled to the control mechanism; a sensor coupled to the outputport of the reaction chamber and to the control unit wherein the sensorgenerates an electrical signal indicative of an electrical parameter ofa liquid outflow from the output port wherein the control unit enables aflow of the first and second liquids into the reaction chamber therebyproducing, continuously, as an outflow at the output port a selectedsalt and wherein the control unit includes circuitry to adjust thecontrol mechanism, so as to maintain the electrical signal within apredetermined range.
 2. An apparatus as in claim 1 wherein the controlunit includes a programmable processor.
 3. An apparatus as in claim 1wherein the control unit includes circuitry for receiving a pH value ofthe selected salt and wherein the control unit includes circuitry forstorage a representation of the pH value.
 4. An apparatus as in claim 3wherein the control circuitry includes circuitry for converting the pHvalue to a corresponding electrical parameter value.
 5. An apparatus asin claim 3 wherein the control unit includes circuitry for compensatingfor temperature variations.
 6. An apparatus as in claim 1 which includesan outflow conduit for transferring the salt directly to a foodprocessing unit.
 7. An apparatus as in claim 1 wherein the sourceincludes an acid storage unit.